The present invention relates to a display system and, particularly, to a display system used in the online mode in which the system is linked to a computer and also in the offline mode in which the system is operated independently of the computer.
Many of the conventional display systems capable of displaying characters, graphics and picture images employ the following method of display. For displaying characters and special symbols coded in 1-byte or 2-byte words (will be called character data hereinafter), the display system incorporates a character generator which generates fonts and symbolic patterns corresponding to the characters and symbols as video signals, which are fed to the picture tube for display in synchronism with the timing of the raster. On the other hand, for command data used to display graphics in the form of vectors (the command data will be called graphic data hereinafter), and for bit-variable data which does not have any of the above-mentioned forms (the bit-variable data will be called image data hereinafter), a full dot memory stores the corresponding or converted dot patterns temporarily in correspondence to the dot position, of the picture tube and, thereafter, the video signal is generated based on the dot patterns in the full dot memory and is fed to the picture tube for display in synchronism with the timing of the raster.
The display system is used in the online mode and in the offline mode. In the online mode, the display system is linked to a host computer so that display data, including character data, graphic data and image data, is displayed on the picture tube of the display system under control of the computer. In the offline mode, the display system is disconnected from the host computer, and display data is entered through an input unit, such as a keyboard associated with the display system, to produce offline pictures which will be stored in the external memory units associated with the display system, or displayed on the picture tube for the purpose of modification or addition by using the keyboard, or outputted to an output unit, such as a printer, associated with the display system.
For online pictures displayed in the online mode, there have been established several standards for the number of characters displayed on one line and the number of characters displayed in one frame. One example of such standards specifies that the character size is 24-by-24 dots, the character pitch is 28 dots along the line, the vertical line pitch is 30 dots, and a 1120-by-720 dot display area is used to display 24 lines by 40 characters/line.
For offline pictures, which are mostly the cases of making documents in the offline mode, character data is usually converted so as to be treated as image data. For example, the character size is 24-by-24 dots and the horizontal character pitch is accorded with the byte boundary such as 32 bits for the convenience of expanding the character fonts on the full dot memory, and the expanded data is contracted to a half in size so that it is stored in the full dot memory at a maximum of 65 characters/line. The character data thus expanded in the form of image data on the full dot memory can be displayed on the picture tube or stored on the disk memory unit associated with the display system.
As mentioned above, online pictures and offline pictures, in many cases, have different display forms such as the character size, horizontal character pitch and vertical character pitch. On this account, online character data stored on a storage medium such as a floppy disk is not readily available for processing in the offline environment. That is, an online picture and an offline picture cannot conveniently be composed on the same screen because of their different display forms, such as the character pitch as mentioned above. One method of solving this problem is that the online character data is converted to the dot patterns of its font which are expanded in the full dot memory as the image data in accordance with the offline picture form and, thereafter, the image data is read out from the full dot memory to be displayed on the picture tube, even during the online operation, instead of displaying the character data directly on the picture tube through hard-wired circuits. This method, however, disadvantageously sacrifices the display speed for character data when the display system is operated under the online environment.